1. Field of the Invention
The present invention relates to a method of retrieving a submerged pipeline. More particularly, the invention relates to a method to remotely sever a damaged submerged pipeline and to retrieve the severed ends of the pipeline to the water surface.
2. Description of the Related Art
Marine pipelines are typically laid on the sea floor from floating vessels called lay barges. To install a submerged pipeline, joints of pipe are welded together on the lay barge. The end of the first joint of pipe is sealed to prevent water from filling the pipeline as the pipeline is laid. As the joints of pipe are welded together into a continuous length of pipeline, the lay barge is moved forward in the water and the welded pipeline slides on pipeline rollers on the lay barge, then over a stinger which is attached to the stern of the lay barge and into the water. As the pipeline leaves the lay barge and the stinger, it bends under its own weight and assumes a generally S-shaped configuration between the stern of the lay barge and the sea floor. The stinger is an extended support which limits the bending of the pipeline between the lay barge and the sea floor, thus reducing the potential of the pipeline buckling as it is being laid. Pipeline tensioners on the deck of the lay barge grip the pipeline and further reduce the bending of the pipeline as it is being laid.
During pipe lay operations of the pipeline, loss of tension in the pipeline due to adverse weather or to failure of the tensioners can cause the pipeline to bend excessively. Excessive bending of the pipeline may cause the pipeline to buckle. If the buckle results in water entering the pipeline it is referred to as a "wet buckle." The submerged pipeline will fill with water, with the pipeline breaking at a location, generally near the stinger, and fall to the sea floor. A pipeline can also be damaged after the pipeline has been laid, for example, by ships dragging their anchors over the pipeline.
Normally, to repair a submerged pipeline which has buckled and broken or has been damaged, the broken or damaged end of the pipeline needs to be lifted to the water surface so that the pipeline can be repaired. Various techniques have been used to raise a submerged pipeline end to the water surface. The depth of the water typically determines whether or not the pipeline end will be recovered using divers or diverless vehicles. Conventional pipeline recovery operations using divers are restricted to maximum depths of about 300 meters. The conventional pipeline recovery technique uses divers to cut the pipeline at the sea floor. The divers then either weld a retrieval line to the severed end of the pipeline or connect a lift head to the pipeline end to raise the pipeline end to the water surface. Lift heads are well known in the art as a means for gripping a pipeline end. The retrieval cable is drawn in by a winch on the lay barge to raise the pipeline end to the water surface. As the pipeline end is raised to the water surface, the section of pipeline raised above the sea floor will bend by its own weight, in the shape of a catenary between the sea floor and the water surface. The pipeline, if filled with water during retrieval of the pipeline to the water surface, is heavy and more likely to buckle than is a lighter pipeline. The pipeline can be lightened by eliminating the water or de-watering the pipeline. A fluid displacing pipeline pig is frequently used to de-water the section of pipeline being raised above the sea floor. Pipeline pigs are typically propelled through a pipeline by injecting a pressurized gas behind the pig.
In deep water, water depths exceeding 300 meters, various techniques have been developed to recover damaged pipelines without sending divers down to the damaged pipeline on the sea floor. A typical recovery method for deep water utilizes a remotely operated vehicle (ROV) which is controlled by operators from a vessel on the water surface. The ROV can manipulate cutting equipment to sever a damaged pipeline. The ROV then inserts a lift head into the open end of the pipeline section which will be raised to the water surface. A retrieval cable attached to the lift head is drawn in by a winch on the lay barge to raise the pipeline end.
Another technique for recovery of a submerged pipeline utilizes a repair frame to grip the submerged pipeline. The repair frame may have cutting equipment to sever the pipeline and a lift head to grip the pipeline attached to the repair frame. In water depths up to 200 meters, divers can operate the repair frame from a control panel located on the repair frame. At depths beyond the reach of divers, a ROV can operate the repair frame from the control panel.
U.S. Pat. No. 4,445,804 to Abdallah et al. discloses a method and apparatus for the remote recovery of submerged pipelines by utilizing a buoy in combination with cables controlled from the lay vessel to deploy equipment to a submerged pipeline. In the '804 patent, a pipe alignment frame is lowered and located in an operational relationship with the submerged pipeline with the aid of a ROV. The pipeline is lifted off the sea floor by an arm of the pipe alignment frame which scoops the pipe up and lifts it. Cutting equipment is then lowered via the cables and stabbed into the pipe alignment frame. The cutting equipment severs the pipeline and is then raised to the lay vessel via the cables. A lift head is lowered via the cables and is stabbed over the end of the pipeline. After the lift head has been actuated to grip the pipeline, the cable is taken in to recover the pipeline to the water surface.
As the offshore oil industry continues to venture into deeper and deeper waters, more and more pipelines are being laid in water depths beyond the range of divers. Due to a variety of factors such as pipeline size, sea floor bearing capacity, sea currents, and lay barge daily rates, a reliable and dependable method for retrieving pipelines either with or without the aid of divers is needed. The method should minimize the amount of handling of the recovered end of the pipeline as it is raised off the sea floor so that the retrieved end can be directly pulled onto the stinger and into the grasp of the pipeline tensioners. Furthermore, the method of retrieving pipelines should minimize cost and minimize dependency on motions of the vessel due to sea conditions, such as heave, roll or pitch, which can result in a significant amount of standby time and cost.